This invention relates to a composite structure in which at least one surface of a polystyrene foam substrate is adhesively bonded to an oriented polypropylene film and to thermoformed articles such as disposable cups, plates, cartons, containers, etc., fabricated from such composite.
Composite structures in which a polymer foam such as a polystyrene foam is faced with a polyolefin sheet or film and thermoformed articles such as containers formed from the composites are known.
U.S. Pat. No. 3,531,367 describes a sandwiched panel of foamed polyethylene and polyolefin sheet. The panel is made by wetting the surface(s) of the polyolefin sheet with a solution of polystyrene in a solvent in which the polystyrene foam is soluble and pressing the wetted surface(s) onto the surface(s) of the foamed polystyrene until a bond is set between the surface(s).
U.S. Pat. No. 4,008,347 describes a laminate of polystyrene foam and unoriented polyolefin film, e.g., unoriented polypropylene film, in which the film is secured to the foam using as a heat-sensitive adhesive a vinylic polymer such as ethylene-vinyl acetate copolymer or vinyl acetate maleate copolymer, or polyamide resin, which is dispersed in an emulsion or solvent carrier. The film is coated with the adhesive emulsion/solvent solution which is permitted to dry before laminating the film to the foam under heat and pressure. The laminate is formed into a receptacle such as a cup, picnic plate, container or closure employing vacuum-thermoforming. The use of oriented polyolefin films, including biaxially oriented film, is avoided in the construction of the laminate since, according to the patent, such a film has a greater tendency to wrinkle and is more difficult to secure firmly to a foam base than unoriented polyolefin film.
U.S. Pat. No. 4,111,349 describes cups, plates, trays or containers obtained by thermoforming a coextruded laminate of a filled, optionally foamed, polystyrene possessing at least one layer of a polyolefin, preferably polypropylene, bonded thereto employing an adhesive such as ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, ethylene-isobutylate copolymer, chlorinated polyethylene, styrene-butadiene block copolymer, epoxy resin or mixture thereof.
U.S. Pat. No. 4,578,297 describes a heat-resistant laminate capable of being thermoformed to provide a container which will maintain its structural and mechanical properties while subjected to microwave energy. The laminate is obtained by affixing one or both surfaces of a thermoplastic polymer foam, e.g., a polyethylene, polypropylene or polystyrene foam, to a biaxially oriented, opaque film possessing a void-containing polypropylene core layer and a void-free polypropylene skin layer (i.e., a foam as described in U.S. Pat. No. 4,377,616) with or without bonding adhesives. In one non-adhesive lamination operation, a "sandwich" of foam and oriented film surface layers is placed in a mold of desired configuration, e.g., that of a container, with subsequent application of heat and pressure providing the laminate in the form of the finished article. In another non-adhesive lamination procedure, the surface of the foam is superficially melted with the opaque film then being applied thereto. The only adhesive bonding technique mentioned employs an adhesive based on ethylene-vinyl acetate copolymer.
Several of these disclosures, e.g., U.S. Pat. Nos. 4,008,347 and 4,036,675, recognize the difficulty of achieving good bond strength between a polystyrene foam substrate and a polyolefin film, particularly oriented polypropylene film.
It is an object of the present invention to provide a polystyrene foam/oriented polypropylene film composite demonstrating high bond strength between the foam and film components of the structure and exhibiting improved microwaveability.
It is yet another object of the invention to provide a simple, economical method for manufacturing such a polystyrene foam/oriented polypropylene film composite and thermoformed articles therefrom.